In a squirrel-cage rotor for an induction motor, a laminated core made by stacking magnetic materials, such as silicon steel plates, is provided with a plurality of slots for disposing secondary conductors therein. These slots axially extend along the outer circumferential surface of the laminated core, and are arranged at generally equal intervals in a circumferential direction. Particularly, in a high-speed induction motor used as, e.g., a spindle motor of a machine tool, the slots of a rotor are formed as axial through holes which do not open at the outer circumferential surface of a laminated core. The slots shaped as through holes in a conventional squirrel-cage rotor are generally provided with a circular profile, or a generally elliptical profile of which a major axis is oriented in the radial direction of the laminated core, in a section orthogonal to the axis of the rotor.
When the squirrel-cage rotor rotates at high speed, stress due to centrifugal force is caused in the laminated core. This stress is concentrated particularly on the minimum radius portions in relation to a center of rotation, i.e., the innermost radial portions, of the walls of the slots of the laminated core. If such a stress concentration exceeds the elastic limit of the material forming the laminated core, a permanent set or a crazing arises on the radial innermost portions of the slot walls of the laminated core. The degree of the stress concentration becomes larger when the crossing angle of the tangents on two given points adjacent to the radial innermost portion of each slot wall becomes smaller in a section orthogonal to the axis.
Even if the slots have sectional shapes which easily lead to stress concentration on the radial innermost portions of the slot walls, the stress concentration caused on the slot walls during the high-speed rotation of the rotor does not become very problematic, as long as the diameter of the laminated core is relatively small. However, in the case of a squirrel-cage rotor of a high-power induction motor, having a laminated core with a larger diameter, is rotated at high speed, the problem arises in the slots having a circular section or a generally elliptical section of which a major axis is oriented in the radial direction of the laminated core, that the stress concentration caused on the slot walls may damage the laminated core.